Ultraviolet light-induced pyrimidine dimers produce death and cause mutations in microorganisms. Although dimers have been implicated in the induction of cancer, their true role is not yet clear. Several pathways for repair of DNA containing pyrimidine dimers exist. In photoreactivation, the photoreactivating enzyme binds specifically to a region of DNA containing a pyrimidine dimer, and on absorption of a photon, the dimer is monomerized and the DNA repaired. The specificity of photoreactivation for pyrimidine dimers allows evaluation of their role in producing biological damage. Using a high yield E. coli strain and a new assay for pyrimidine dimers, I shall study the sub-unit structure of the enzyme by denaturing the enzyme and electrophoresing it on polyacrylamide gels, and by ultracentrifugation. I plan to identify the chromophore by its optical and chemical properties, reconstitution with the protein and isolation from known radioactive precursors. I shall also study the mechanism of photoreactivation.